Field of the Invention
The present invention relates to an image forming apparatus employing an electrophotographic image forming system.
Description of the Related Art
An image forming apparatus such as a printer employing an electrophotographic image forming system (an electrophotographic process) uniformly charges an electrophotographic photosensitive member (hereinafter called a “photosensitive member”) serving as an image bearing member and selectively exposes the charged photosensitive member to form an electrostatic latent image on the photosensitive member. The electrostatic latent image formed on the photosensitive member is visualized as a toner image by toner serving as a developer. Then, the toner image formed on the photosensitive member is transferred onto a recording member such as a recording sheet and a plastic sheet. After that, heat or pressure is applied to the toner image transferred onto the recording member to fix the toner image onto the recording member for image recording.
Such an image forming apparatus also employs, in order to facilitate various maintenance operations, a process cartridge system in which a photosensitive member and a processing section acting on the photosensitive member are integrated into a cartridge and the cartridge is attachable/detachable to/from the body of an electrophotographic image forming apparatus. Meanwhile, an electrostatic latent image described above changes its electrical characteristics according to use conditions. Particularly, the sensitivity of a photosensitive member changes due to the abrasion of the film thickness of a charge transport layer that retains exposure history or an electrostatic potential received by the photosensitive member in use. As a result, it is known that image density, particularly, an image having various density regions such as a graphic image changes.
Japanese Patent Application Laid-open No. H5-66638 discloses a method for maintaining the density of an image at a uniform level in such a way that a surface electrometer is embedded in an image forming apparatus as a section that detects potential information on a photosensitive member and fluctuations in a potential with the use of the photosensitive member are detected and corrected.
Further, Japanese Patent Application Laid-open No. 2013-125097 and Japanese Patent Application Laid-open No. 2012-13381 propose a method for easily detecting a potential of a photosensitive member in such a way that a discharge start voltage applied from a charging member and a transfer member to the photosensitive member is measured instead of a direct measurement method such as a surface electrometer as a method for detecting a potential of another photosensitive member.
In recent years, there has been a demand for reduction in cost per page (CPP) for electrophotographic image forming apparatuses and process cartridges in the market. Accordingly, both cost reduction and long service life are required to coexist in the image forming apparatuses and the process cartridges. As one of problems residing in the long service life of such an apparatus, an exposure potential changes from the initial use of a photosensitive member to the latter part of the service life with an increase in an exposure amount received by the photosensitive member. In addition, a charge transport layer greatly changes its film thickness from an initial film thickness adapted to a long service life to a thin film thickness due to abrasion after the long use of a photosensitive member, which results in fluctuations in an exposure potential. Particularly, a middle tone potential from a charge potential to an exposure potential greatly fluctuates. Therefore, there is a case that graphic images or the like may change.
Further, in order to correct a middle tone, there has been proposed a method for creating detection toner on an image bearing member such as a photosensitive member. Such a method is effective in that it allows the output of the density of a middle tone with high reproducibility from the initial use of an apparatus to the latter part of a service life. However, since a detection toner image is formed on the image bearing member to perform correction control, it takes time to perform the formation of the toner image, cleaning after the detection of the toner image, or the like. In addition, since toner is used to form the detection toner image, the toner is necessarily consumed every time the correction control is performed.